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Meteor fireball. Illustration of a fireball forming as a large meteoroid (originally a small asteroid) heats up as it passes through Earth's atmosphere. The meteoroid may explode in the air (an airburst), or it may survive to impact the ground as a meteorite. The effect of a large meteorite impact or meteor airburst over a city would be catastrophic, with extensive damage and loss of life. Asteroids that are 30 to 40 metres across are large enough to wipe out a city. Around a million such asteroids exist, though unlike the larger asteroids that could cause global destruction, only a small number of these 'city killers' are being tracked.